Overexpression of Banana ATG8f Modulates Drought Stress Resistance in Arabidopsis

The lowdown on breakdown: Open questions in plant proteolysis

Proteolysis, including post-translational proteolytic processing as well as protein degradation and amino acid recycling, is an essential component of the growth and development of living organisms. In this article, experts in plant proteolysis pose and discuss compelling open questions in their areas of research. Topics covered include the role of proteolysis in the cell cycle, DNA damage response, mitochondrial function, the generation of N-terminal signals (degrons) that mark many proteins for degradation (N-terminal acetylation, the Arg/N-degron pathway, and the chloroplast N-degron pathway), developmental and metabolic signaling (photomorphogenesis, abscisic acid and strigolactone signaling, sugar metabolism, and postharvest regulation), plant responses to environmental signals (endoplasmic-reticulum-associated degradation, chloroplast-associated degradation, drought tolerance, and the growth-defense trade-off), and the functional diversification of peptidases. We hope these thought-provoking discussions help to stimulate further research.

The Role of Plant Ubiquitin-like Modifiers in the Formation of Salt Stress Tolerance

The climate-driven challenges facing Earth necessitate a comprehensive understanding of the mechanisms facilitating plant resilience to environmental stressors. This review delves into the crucial role of ubiquitin-like modifiers, particularly focusing on ATG8-mediated autophagy, in bolstering plant tolerance to salt stress. Synthesising recent research, we unveil the multifaceted contributions of ATG8 to plant adaptation mechanisms amidst salt stress conditions, including stomatal regulation, photosynthetic efficiency, osmotic adjustment, and antioxidant defence. Furthermore, we elucidate the interconnectedness of autophagy with key phytohormone signalling pathways, advocating for further exploration into their molecular mechanisms. Our findings underscore the significance of understanding molecular mechanisms underlying ubiquitin-based protein degradation systems and autophagy in salt stress tolerance, offering valuable insights for designing innovative strategies to improve crop productivity and ensure global food security amidst increasing soil salinisation. By harnessing the potential of autophagy and other molecular mechanisms, we can foster sustainable agricultural practices and develop stress-tolerant crops resilient to salt stress.

Systematic analysis and expression of Gossypium ATG8 family reveals the roles of GhATG8f responding to salt stress in cotton

KEY MESSAGE

Comprehensive analysis of Gossypium ATG8 family indicates that GhATG8f could improve salt tolerance of cotton by increasing SOD, POD and CAT activity and proline accumulation. In plants, autophagy is regulated by several genes that play important roles in initiating and controlling the process. ATG8, functioning as a protein similar to ubiquitin, is involved in crucial tasks throughout the autophagosome formation process. In this research, we conducted an extensive and all-encompassing investigation of 64 ATG8 genes across four varieties of cotton. According to the subcellular localization prediction results, 49 genes were found in the cytoplasm, 6 genes in the chloroplast, 1 gene in the peroxisome, 5 genes in the nucleus, and 3 genes in the extracellular region. Phylogenetic analysis categorized a total of 5 subfamilies containing sixty-four ATG8 genes. The expression of the majority of GhATG8 genes was induced by salt, drought, cold, and heat stresses, as revealed by RNA-seq and real-time PCR. Analysis of cis-elements in the promoters of GhATG8 genes revealed the predominant presence of responsive elements for plant hormones and abiotic stress, suggesting that GhATG8 genes might have significant functions in abiotic stress response. Furthermore, we additionally performed a gene interaction network analysis for the GhATG8 proteins. The salt stress resistance of cotton was reduced due to the downregulation of GhATG8f expression, resulting in decreased activity of CAT, SOD, and POD enzymes, as well as decreased fresh weight and proline accumulation. In summary, our research is the initial exploration of ATG8 gene components in cotton, providing a basis for future investigations into the regulatory mechanisms of ATG8 genes in autophagy and their response to abiotic stress.

The apple autophagy-related gene MdATG10 improves drought tolerance and water use efficiency in transgenic apple plants

The Loess Plateau is the main apple production area in China; low precipitation is one of the most important factors limiting apple production here. Autophagy is a conserved process in eukaryotes that recycles cell contents or damaged macromolecules. Previously, we identified an autophagy-related gene MdATG10 from apple plants, which was involved in the responses to stressed conditions. In this study, we found that MdATG10 improved the drought tolerance and water use efficiency (WUE) of transgenic apple plants. MdATG10-overexpressing (OE) apple plants were more tolerant of short-term drought stress, as evidenced by their fewer drought-related injuries, compared with wild-type (WT) apple plants. In addition, the WUE of OE plants was higher than that of WT plants under long-term moderate water deficit conditions. The growth rate, biomass accumulation, photosynthetic efficiency, and stomatal aperture were higher in OE plants than in WT plants under long-term moderate drought conditions. During the process of adapting to drought, the expressions of genes involved in the abscisic acid (ABA) pathway were reduced in OE plants to decrease the synthesis of ABA, which helped maintain the stomatal opening for gas exchange. Furthermore, autophagic activity was higher in OE plants than in WT plants, as evidenced by the higher expressions of ATG genes and the greater number of autophagy bodies. In sum, our results suggested that overexpression of MdATG10 improved drought tolerance and WUE in apple plants, possibly by regulating stomatal movement and enhancing autophagic activity, which then enhanced the photosynthetic efficiency and reduced damage, as well as the reactive oxygen species (ROS) accumulation in apple plants.

Redox Regulation of Protein Functioning

Reactive oxygen species (ROS) and their derivatives play a key role in signaling under normal and oxidative stress conditions in all aerobic living organisms [...].

Alfalfa MsATG13 Confers Cold Stress Tolerance to Plants by Promoting Autophagy

Autophagy is a conserved cellular process that functions in the maintenance of physiological and metabolic balance. It has previously been demonstrated to improve plant tolerance to abiotic stress. Numerous autophagy-related genes (ATGs) that regulate abiotic stress have been identified, but there have been few functional studies showing how ATGs confer cold stress tolerance. The cold transcriptome data of the crown buds that experienced overwintering of the alfalfa (Medicago sativa L.) showed that MsATG13 is upregulated in response to cold stress. In the present study, we found that MsATG13 transgenic tobacco enhanced cold tolerance compared to wild-type (WT) plants. Transmission electron microscopy demonstrated that transgenic tobacco overexpressing MsATG13 formed more autophagosomes than WT plants in response to cold stress conditions. The transgenic tobacco increased autophagy levels due to upregulation of other ATGs that were necessary for autophagosome production under cold stress conditions. MsATG13 transgenic tobacco also increased the proline contents and antioxidant enzyme activities, enhancing the antioxidant defense capabilities under cold stress conditions. Furthermore, MsATG13 overexpression decreased levels of superoxide anion radicals and hydrogen peroxide under cold stress conditions. These findings demonstrate the role of MsATG13 in enhancing plant cold tolerance through modulation of autophagy and antioxidant levels.

Effects of polystyrene nanoplastics on the physiological and biochemical characteristics of microalga Scenedesmusquadricauda

The contamination of the aquatic environment with microplastics has become a global environmental concern. Microplastic particles can be shredded to form smaller nanoplastics, and knowledge on their impacts on phytoplankton, especially freshwater microalgae, is still limited. To investigate this issue, the microalga Scenedesmus quadricauda was exposed to polystyrene nanoplastics (PS-NPs) of five concentrations (10, 25, 50, 100, and 200 mg/L). The growth; the contents of antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD); the chlorophyll content; and concentrations of soluble protein and soluble polysaccharide were accordingly measured. The results showed that the microalgal density increased with the increase of the polystyrene nanoplastic concentrations, and the physiological features of alga were enhanced after the stimulation of nanoplastics. Furthermore, a high concentration (200 mg/L) of nanoplastics increased the contents of chlorophyll, soluble protein, and polysaccharide (P < 0.05). The antioxidant enzyme activities of Scenedesmus quadricauda were significantly activated by nanoplastics. Lastly, we propose three possible algal recovery mechanisms in response to nanoplastics in which Scenedesmus quadricauda was tolerant with PS-NPs by cell wall thickening, internalization, and aggregation. The results of this study contribute to understanding of the ecological risks of nanoplastics on freshwater microalgae.

Genome-Wide Identification of ATG Gene Family Members in Fagopyrum tataricum and Their Expression during Stress Responses

Abiotic stresses such as drought and salinity are major environmental factors limiting plant productivity. Autophagy-related genes are extensively involved in plant growth, development, and adverse stress responses, which have not yet been characterized in Tartary buckwheat (Fagopyrum tataricum, TB). In this study, we verified that drought stress could induce autophagy in TB roots. Next, 49 FtATGs in the whole genome of TB were identified. All FtATGs were randomly distributed in 8 known chromosomes, while 11 FtATGs were predictably segmental repeats. As the core component of autophagy, there were 8 FtATG8s with similar gene structures in TB, while FtATG8s showed high expression at the transcription level under drought and salt stresses. The cis-acting element analysis identified that all FtATG8 promoters contain light-responsive and MYB-binding elements. FtATG8s showed a cell-wide protein interaction network and strongly correlated with distinct stress-associated transcription factors. Furthermore, overexpression of FtATG8a and FtATG8f enhanced the antioxidant enzyme activities of TB under adverse stresses. Remarkably, FtATG8a and FtATG8f may be vital candidates functioning in stress resistance in TB. This study prominently aids in understanding the biological role of FtATG genes in TB.

Autophagy: a game changer for plant development and crop improvement

Manipulation of autophagic pathway represents a tremendous opportunity for designing climate-smart crops with improved yield and better adaptability to changing environment. For exploiting autophagy to its full potential, identification and comprehensive characterization of adapters/receptor complex and elucidation of its regulatory network in crop plants is highly warranted.  Autophagy is a major intracellular trafficking pathway in eukaryotes involved in vacuolar degradation of cytoplasmic constituents, mis-folded proteins, and defective organelles. Under optimum conditions, autophagy operates at a basal level to maintain cellular homeostasis, but under stressed conditions, it is induced further to provide temporal stress relief. Our understanding of this highly dynamic process has evolved exponentially in the past few years with special reference to several plant-specific roles of autophagy. Here, we review the most recent advances in the field of autophagy in plants and discuss its potential implications in designing crops with improved stress and disease-tolerance, enhanced yield potential, and improved capabilities for producing metabolites of high economic value. We also assess the current knowledge gaps and the possible strategies to develop a robust module for biotechnological application of autophagy to enhance bioeconomy and sustainability of agriculture.

Autophagy during drought: function, regulation, and potential application

Drought is a major challenge for agricultural production since it causes substantial yield reduction and economic loss. Autophagy is a subcellular degradation and recycling pathway that functions in plant development and responses to many stresses, including drought. In this review, we summarize the current understanding of the function of autophagy and how autophagy is upregulated during drought stress. Autophagy helps plants to survive drought stress, and the mechanistic basis for this is beginning to be elucidated. Autophagy can selectively degrade aquaporins to adjust water permeability, and also degrades excess heme and damaged proteins to reduce their toxicity. In addition, autophagy can degrade regulators or components of hormone signaling pathways to promote stress responses. During drought recovery, autophagy degrades drought-induced proteins to reset the cell status. Autophagy is activated by multiple mechanisms during drought stress. Several transcription factors are induced by drought to upregulate autophagy-related gene expression, and autophagy is also regulated post-translationally through protein modification and stability. Based on these observations, manipulation of autophagy activity may be a promising approach for conferring drought tolerance in plants.

Genome-wide analysis of autophagy-related genes in Medicago truncatula highlights their roles in seed development and response to drought stress

Autophagy is a highly conserved process of degradation of cytoplasmic constituents in eukaryotes. It is involved in the growth and development of plants, as well as in biotic and abiotic stress response. Although autophagy-related (ATG) genes have been identified and characterized in many plant species, little is known about this process in Medicago truncatula. In this study, 39 ATGs were identified, and their gene structures and conserved domains were systematically characterized in M. truncatula. Many cis-elements, related to hormone and stress responsiveness, were identified in the promoters of MtATGs. Phylogenetic and interaction network analyses suggested that the function of MtATGs is evolutionarily conserved in Arabidopsis and M. truncatula. The expression of MtATGs, at varied levels, was detected in all examined tissues. In addition, most of the MtATGs were highly induced during seed development and drought stress, which indicates that autophagy plays an important role in seed development and responses to drought stress in M. truncatula. In conclusion, this study gives a comprehensive overview of MtATGs and provides important clues for further functional analysis of autophagy in M. truncatula.

MdATG5a induces drought tolerance by improving the antioxidant defenses and promoting starch degradation in apple

Drought occurrence seriously affects the productivity and quality of apple crop worldwide. Autophagy, a conserved process for the degradation and recycling of unwanted cellular components, is considered to positively regulate the tolerance of various abiotic stresses in plants. In the current study, we isolated two ATG5 homologs genes, namely, MdATG5a and MdATG5b, from apple, demonstrating their responsiveness to drought and oxidative stresses. In addition to having the same cellular localization in the nucleus and cytoplasm, both MdATG5a and MdATG5b could interact with MdATG12. Transgenic apple plants overexpressing MdATG5a exhibited an improved drought tolerance, as indicated by less drought-related damage and higher photosynthetic capacities compared to wild-type (WT) plants under drought stress. The overexpression of MdATG5a improved antioxidant defenses in apple when exposed to drought via elevating both antioxidant enzyme activities and the levels of beneficial antioxidants. Furthermore, under drought stress, the overexpression of MdATG5a promoted the mobilization of starch to accumulate greater levels of soluble sugars, contributing to osmotic adjustments and supporting carbon skeletons for proline synthesis. Such changes in physiological responses may be associated with increased autophagic activities in the transgenic plants upon exposure to drought. Our results demonstrate that MdATG5a-mediated autophagy enhances drought tolerance of apple plants via improving antioxidant defenses and metabolic adjustments.

Multi-omics analyses on the response mechanisms of 'Shine Muscat' grapevine to low degree of excess copper stress (Low-ECS)

Copper stress is one of the most severe heavy metal stresses in plants. Grapevine has a relatively higher copper tolerance than other fruit crops. However, there are no reports regarding the tolerance mechanisms of the 'Shine Muscat' ('SM') grape to a low degree of excess copper stress (Low-ECS). Based on the physiological indicators and multi-omics (transcriptome, proteome, metabolome, and microRNAome) data, 8 h (h) after copper treatment was the most severe stress time point. Nonetheless, copper stress was alleviated 64 h after treatment. Cu ion transportation, photosynthesis pathway, antioxidant system, hormone metabolism, and autophagy were the primary response systems in 'SM' grapevine under Low-ECS. Numerous genes and proteins, such as HMA5, ABC transporters, PMM, GME, DHAR, MDHAR, ARGs, and ARPs, played essential roles in the 'SM' grapevine's response to Low-ECS. This work was carried out to gain insights into the multi-omics responses of 'SM' grapevine to Low-ECS. This study provides genetic and agronomic information that will guide better vinery management and breeding copper-resistant grape cultivars.

Control of ABA Signaling and Crosstalk with Other Hormones by the Selective Degradation of Pathway Components

A rapid and appropriate genetic and metabolic acclimation, which is crucial for plants' survival in a changing environment, is maintained due to the coordinated action of plant hormones and cellular degradation mechanisms influencing proteostasis. The plant hormone abscisic acid (ABA) rapidly accumulates in plants in response to environmental stress and plays a pivotal role in the reaction to various stimuli. Increasing evidence demonstrates a significant role of autophagy in controlling ABA signaling. This field has been extensively investigated and new discoveries are constantly being provided. We present updated information on the components of the ABA signaling pathway, particularly on transcription factors modified by different E3 ligases. Then, we focus on the role of selective autophagy in ABA pathway control and review novel evidence on the involvement of autophagy in different parts of the ABA signaling pathway that are important for crosstalk with other hormones, particularly cytokinins and brassinosteroids.

Knowing When to Self-Eat - Fine-Tuning Autophagy Through ATG8 Iso-forms in Plants

Autophagy is a catabolic process that takes place under both normal and adverse conditions and is important for the degradation of various organelles and proteins that are no longer needed. Thus, it can be viewed as both a constitutive recycling machinery and an adaptation mechanism. Increase in the activity of autophagy can be caused by multiple biotic and abiotic stress factors. Though intensive research in the past decade has elucidated many molecular details of plant autophagy, the mechanisms of induction and regulation of the process remain understudied. Here, we discuss the role of ATG8 proteins in autophagic signaling and regulation with an emphasis on the significance of ATG8 diversification for adapting autophagy to the changing needs of plants.

Melatonin synthesis enzymes interact with ascorbate peroxidase to protect against oxidative stress in cassava

Melatonin is an important indole amine hormone in animals and plants. The enzymes that catalyse melatonin synthesis positively regulate plant stress responses through modulation of the accumulation of reactive oxygen species (ROS). However, the relationship between melatonin biosynthetic enzymes and ROS-scavenging enzymes has not been characterized. In this study, we demonstrate that two enzymes of the melatonin synthesis pathway in Manihot esculenta (MeTDC2 and MeASMT2) directly interact with ascorbate peroxidase (MeAPX2) in both in vitro and in vivo experiments. Notably, in the presence of MeTDC2 and MeASMT2, MeAPX2 showed significantly higher activity and antioxidant capacity than the purified MeAPX2 protein alone. These findings indicate that MeTDC2-MeAPX2 and MeASMT2-MeAPX2 interactions both activate APX activity and increase antioxidant capacity. In addition, the combination of MeTDC2, MeASMT2, and MeAPX2 conferred improved resistance to hydrogen peroxide in Escherichia coli. Moreover, this combination also positively regulates oxidative stress tolerance in cassava. Taken together, these findings not only reveal a direct interaction between MeTDC2, MeASMT2, and MeAPX2, but also highlight the importance of this interaction in regulating redox homoeostasis and stress tolerance in cassava.

Comprehensive Analysis of Autophagy-Related Genes in Sweet Orange (Citrus sinensis) Highlights Their Roles in Response to Abiotic Stresses

Autophagy is a highly conserved intracellular degradation pathway that breaks down damaged macromolecules and/or organelles. It is involved in plant development and senescence, as well as in biotic and abiotic stresses. However, the autophagy process and related genes are largely unknown in citrus. In this study, we identified 35 autophagy-related genes (CsATGs—autophagy-related genes (ATGs) of Citrus sinensis, Cs) in a genome-wide manner from sweet orange (Citrus sinensis). Bioinformatic analysis showed that these CsATGs were highly similar to Arabidopsis ATGs in both sequence and phylogeny. All the CsATGs were randomly distributed on nine known (28 genes) and one unknown (7 genes) chromosomes. Ten CsATGs were predicted to be segmental duplications. Expression patterns suggested that most of the CsATG were significantly up- or down-regulated in response to drought; cold; heat; salt; mannitol; and excess manganese, copper, and cadmium stresses. In addition, two ATG18 members, CsATG18a and CsATG18b, were cloned from sweet orange and ectopically expressed in Arabidopsis. The CsATG18a and CsATG18b transgenic plants showed enhanced tolerance to osmotic stress, salt, as well as drought (CsATG18a) or cold (CsATG18b), compared to wild-type plants. These results highlight the essential roles of CsATG genes in abiotic stresses.